We had a problem last weekend with Postfix not accepting email for a single domain when it was coming from outside our network, while messages from hosts on the local network were accepted and routed with no problems. Messages from outside the network were rejected with a 450 (temporary) code and the error message “Recipient address rejected: Domain not found”. The cause did end up being a DNS problem (apparently the most common kind of issue with Postfix), but not one that I would have expected (a missing host entry for the top-level domain, so example.com wouldn’t resolve even though mail.example.com did). Finding the source of the problem was complicated because of a set of several changes during a weekend maintenance window.